It's like a dog whistle. It's emblazoned on my brain. You all know what I'm talking about. Unless you're way too young and you're intent on making me feel way too old. I'm talking, of course, about the noises your dial-up modem made when hooking up to the internet back when the internet was this terrifying thing we used to ‘surf'.

The above massive image attempts to visualise every aspect of that modem noise, and also does a great job of explaining precisely what every blip and bloop meant, what it represented. I just love it.

Oona Räisänen does her level best to explain, but really you should just click on the image below…

When humans talk, only one of them is usually talking while the other one listens. The telephone network exploits this fact and temporarily silences the return channel to suppress any confusing echoes of the talker's own voice.

Modems don't like this at all, as they can very well talk at the same time (it's called full-duplex). The answering modem now puts on a special answer tone that will disable any echo suppression circuits on the line. The tone also has periodic "snaps" (180° phase transitions) that aim to disable yet another type of circuit called echo canceller.

Now the modems will list their supported modulation modes and try to find one that both know. They also probe the line with test tones to see how it responds to tones of different frequencies, and how much it attenuates the signal. They exchange their test results and decide a speed that is suitable for the line.